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Even though concrete is a very rigid material, it has a natural weakness when it comes to tension. It is limited with respect to the length of a beam, floor, or bridge that can be made out of it. One way to be able to build these structures with longer spans than would be possible with ordinary concrete is through a technique called pre-stressing. A post-tension slab is a slab of concrete that has been pre-stressed using a specific method to increase the strength of the concrete.
Several methods exist for pre-stressing concrete, with post-tensioning being a very common one. Before a post-tension slab is poured, high-strength steel strands or cables, called tendons, are laid in a tight grid. These help support and give strength to the slab once it has cured. The tendons are sheathed in plastic so that they do not directly touch the concrete. After the grid is made, the concrete is poured, with extra care taken to make sure that the tendons remain at the correct depth.
The concrete is allowed to cure to about 75% of the way, at which point post-tensioning occurs. Each of the tendons in the post-tension slab is pulled tight, using a hydraulic jack. The tensing of the cables occurs after the concrete has mostly cured, hence the term “post-tension.” The tendons are usually pulled to a tension of 25,000 pounds per square inch (4503 kg per square cm). Once the cables have reached the designated tension, they are anchored in the concrete, and the slab is allowed to fully cure.
Many modern homes are built on a post-tension slab, which serves as an excellent foundation. This method of pre-stressing concrete is especially useful in areas where the soil expands and contracts relative to weather conditions. Apart from residential applications, post-tension concrete opens up the possibility of many construction techniques that otherwise would be impossible. For example, parking garages and stadiums are stronger and cheaper to construct with post-tension concrete.
Using a post-tension slab rather than ordinary concrete often makes good economic sense. Because there is a smaller depth of concrete used to obtain the same end result, construction costs are reduced. This particular advantage has even larger implications for the construction of skyscrapers and office buildings.
When the floor thickness is reduced, so is the weight of the structure. A lighter building means that the cost of building the foundation is reduced. Thinner floors also translate into reduced building height, which means that exterior finishing costs, such as window glass, are lessened.